Magneto-inductive flow measuring systems are often used in plants of process and/or automation technology for determining volume flow or mass flow. For volumetric flow measurement, magneto-inductive flow measuring systems utilize the principle of electrodynamic induction: charge carriers of the measured substance moved perpendicularly to a magnetic field induce a voltage in electrodes likewise arranged essentially perpendicular to the flow direction of the measured substance and perpendicular to the direction of the magnetic field. The measurement voltage induced in the measured substance and accessed by means of the electrodes is proportional to the flow velocity of the measured substance averaged over the cross section of the measuring tube, and thus to the volume flow rate. If the density of the measured substance is known, the mass flow in the pipeline or the measuring tube can be determined. The measurement voltage is usually accessed via the electrode pair, which is arranged in the measuring tube region, in which the maximum magnetic field strength, and thus the maximum measurement voltage, is to be expected. The electrodes are usually galvanically coupled with the measured substance; however, magneto-inductive flow measuring systems with contactless, capacitively coupling electrodes are also known.
The measuring tube can be manufactured either from an electrically conductive material, e.g. stainless steel, or it can be composed of an electrically insulating material. If the measuring tube is manufactured of an electrically conductive material, it must then be lined in the region coming in contact with the measured substance with a lining—the so-called liner—made of an electrically insulating material. The liner is, depending on temperature and measured substance, composed, for example, of a thermoplastic, a thermosetting or an elastomeric synthetic material. However, magneto-inductive flow measuring systems with a ceramic lining are also known.
In the case of use of electrodes contacting a measured substance, at the interface between the electrode and the measured substance flowing through the measuring tube, galvanic elements form, which bring about an electrochemical disturbance potential. This electrochemical disturbance potential is variable, and dependent on different, changing, environmental conditions, such as temperature, pressure, composition of the measured substance, material of the electrodes and material of the measuring tube. It is evident that an electrochemical potential which changes over time negatively influences the accuracy of measurement of a conventional magneto-inductive flow measuring system. Methods have therefore become known for eliminating these disturbance signals. Galvanic elements which bring about an electrochemical disturbance potential can also arise due to accretion deposited from the measured substance onto the electrodes or onto the lining. Such accretion can additionally lead to corruption of the measured values.
In Offenlegungsschrift DE 102006033112 A1, a method is disclosed for operating a flow measuring device, especially for determining the electrical conductivity of the measured substance flowing through the measuring tube. For such purpose, a first electrode is supplied with a signal in the form of an electrical current or a voltage, and at a second electrode, which is not supplied with the signal, impedances, electrical currents or voltages are ascertained. From a comparison of currently measured values with stored values, the electrical conductivity of the measured substance is determined, and/or an accretion formation on the electrodes is recognized.
Another method is known from EP 1536211 A1. There, for determining, among other things, the electrical conductivity of the measured substance, resistance values gained by means of a so-called 2-point and/or 3-point measuring are compared with one another. The 2-point or 3-point measuring occurs, in such case, by means of measuring electrodes and at least one additional electrode.
Currently known methods have the disadvantage that the measuring range for determining the electrical conductivity of a measured substance is limited, since, due to polarisation phenomena, the impedance measured on the electrodes starting from a conductivity of about 1 mS/cm is strongly frequency-dependent and complex.
Such polarization effects occur when an electrical current flows through the interface between the measured substance and electrode, since charge carriers present in the measured substance are deposited on the electrode, and thus influence the measurement voltage.